Extensional viscosity of a thermotropic liquid crystalline polymer

The transient extensional viscosity of the nematic melt of thermotropic liq uid crystalline (LCP) copolyester Vectra A950 was measured in uniaxial exte nsional flow in a commercial constant strain rate rheometer. For extension rates between 0.005 and 1 s(-1) the extensional viscosity of Vectra A950 do es not reach steady state for Hencky strains up to three to four units, whe re most samples break. If the viscosity at a certain value of strain is plo tted versus the strain rate, a rate-thinning curve is obtained. The extensi onal viscosity is much higher than three times the sheer viscosity over a b road range of accumulated strain, while injection molded samples always sho w higher viscosity than compressed samples. The theory developed by Larson and Doi [Larson, R. L. and M. Doi, J. Rheol. 35, 539-563 (1991)] to describ e the flow of polydomain LCPs has been adapted for use in elongational flow . The trends of the predictions agree with the measured stress growth curve s. The how seems to be dominated by the effect of the texture. The variatio n in the viscosity measured in samples prepared by different methods is att ributed to the variation in the liquid crystalline structure present in the samples and in particular to the initial domain size. (C) 2000 The Society of Rheology: [S0148-6055(00)00505-8].